There has been an approach to decrease rolling resistance of a tire to suppress heat generation and thus achieve fuel economy of a vehicle. The demand for achieving fuel economy of a vehicle from the aspect of tires has increased in recent years. It has been desired to achieve fuel economy from the aspect of improvement of an inner liner which requires high air impermeability as well as a tread which occupies a large part of a tire. Examples of known methods for achieving low heat build-up of a rubber composition for an inner liner include a method of using a low-reinforcing filler, and a method of reducing the reinforcing filler content. Also, an attempt has been made to achieve the low heat build-up by using silica as a filler.
However, the above methods for achieving fuel economy (low heat build-up) from the aspect of fillers problematically decrease the hardness of the rubber composition, which softens the tire and decreases the flex fatigue resistance. Hence, it has been difficult to achieve both excellent low heat build-up and high flex fatigue resistance.
Meanwhile, natural rubber has been widely used for inner liners. Natural rubber has a higher Mooney viscosity than those of other synthetic rubbers and thus has low processability. Therefore, natural rubber to be used is usually added with a peptizer and then masticated so that the rubber has a decreased Mooney viscosity. Requirement of such a process in the case of using natural rubber decreases the productivity. Further, the mastication causes molecular chain scission in natural rubber, thereby problematically leading to a loss of the properties of a high-molecular-weight polymer that natural rubber essentially has (for example, high abrasion resistance, fuel economy, and rubber strength).
Patent Document 1 discloses a rubber composition prepared using natural rubber and an epoxidized natural rubber so as to increase the content of non-petroleum resources. However, the rubber composition still has room for improvement in achieving good air impermeability while achieving both low heat build-up and flex crack growth resistance, and in simultaneously improving processability.    Patent Document 1: JP 2007-169431 A